Generally, when interconnecting integrated circuits (ICs), output drivers are used to drive the output signal of one IC to be received by another IC, as illustrated in FIG. 1. A conventional IC 1 generates an IC signal and an output driver 2 receives the IC signal and generates a corresponding output signal that can be received and read by another IC.
For high-speed IC designs, a common type of output driver 2 is a serial line driver, which receives the IC signal and drives the output signal in a serial manner. Turning to FIG. 2, a portion of a conventional serial line driver 10 is shown. The driver 10 includes two n-channel metal-oxide semiconductor field-effect transistors (NMOSs) 50, which receive the IC signal at the gates 60 of the transistors 50 as a differential input signal, i.e., the input signal is the voltage difference between the gates 60. In the case of an IC 1 generating digital signals, e.g., 1""s and 0""s, the input signal may be either a high voltage on one gate 60 and a low voltage on the other gate 60 or vice versa and may swing between the two values depending on the signal that the IC 1 generates.
The output signal 40 is generated as a differential output signal at the drains 35 of the transistors 50xe2x80x94the output signal is the termination voltage difference between the drains 35. In the case of an IC 1 generating digital signals, the output signal may swing between a first value and a second value depending on the input signal, as described above. A common power supply 20 provides the termination voltage at the drains 35 through two equal valued resistors 30xe2x80x94typically around 50 ohmsxe2x80x94one at each drain 35. A common current source 70, represented as a transistor, is coupled to the sources 45 of the transistors 50 and controls the voltage output swing of the output signal 40xe2x80x94the peak-to-peak voltage of the output signal 40.
There are several different industry input/output (I/O) standards for the differential output signal 40, such as the low voltage differential signaling (LVDS) and the low voltage transistor to transistor logic (LVTTL), and many of these standards are incompatible with each other. For example, output signals 40 conforming to one standard may have a different common mode voltagexe2x80x94referring to FIG. 2, the common mode voltage is the average voltage across the output signal 40, i.e., between the two drains 35xe2x80x94or a different output swing compared to other standards.
Most of the existing output drivers 2 conform only to a single I/O standard. However, there are some applications, such as programmable gate arrays (PGAs) where it may be. desirable to have output drivers that conform to several different standards.
The present invention provides a method and mechanism for driving the output signal of an integrated circuit. According to an embodiment, the common mode voltage and the output swing of a differential output signal generated by an output driver may be programmable.
According to another embodiment, a first power supply is used to supply the termination voltage of the output signal, and a second power supply is used to adjust the common mode voltage.